FMCW radar locating devices are frequently used in driver assistance systems for motor vehicles, for example for automatic distance control or for early recognition of the risk of a collision. A radome that covers the antenna and is mostly formed by a radar lens is exposed to a particularly high degree to weather influences and street dirt, so that a reflective dielectric coating (a film of dirt or water) that causes losses can easily form on the surface of the radome.
This significantly reduces the transmit and receive power of the radar sensor, so that detection depth and reliability are significantly limited, up to the complete blinding of the radar sensor. For example, at a radar frequency of 76.5 GHz, a film of water on the radome having a thickness of 0.3 millimeters already has the result that approximately 50% of the radiated power is reflected by this water film, and approximately 90% of the rest of the power is attenuated by absorption.
A coating on the radome is therefore an example of a “diffuse source of loss” that can significantly impair the sensitivity of the radar locating system.
A further example of a diffuse source of loss is precipitation in the form of rain, spray, snow, or hail that reflects a part of the emitted radar radiation and thus causes a reduction in the range of the radar radiation, thus causing a reduction in the locating depth of the radar sensor.
For reasons of traffic safety, it is essential to determine a blinding or limitation of the function of the radar locating device as quickly as possible.
German Patent Application No. DE 199 45 268 A1 describes a method in which a blinding of the radar locating device is to be determined by monitoring a multiplicity of criteria. One of the criteria is based on an evaluation of the average power of the signals received by the radar locating device. However, a disadvantage of this method is that the average power is a function not only of the presence of diffuse sources of loss, but also of a large number of other factors, including, among others, specific properties of the respective radar sensor, assembly and installation tolerances of the radar sensor when installed in the motor vehicle, and in particular influences of temperature and aging.
German Patent Application No. DE 10 2006 054 320 A1 describes a method for detecting precipitation that is also based on the evaluation of a power characteristic of the received radar signal, and is briefly explained below.
The principle of operation of an FMCW (Frequency Modulated Continuous Wave) radar locating device is that the radar signal is continuously sent out, but the frequency of this signal is periodically modulated with rising and falling ramps (here, the term “ramp” is not intended to mean that the frequency modulation within the “ramp” has to be linear). The mixer mixes a part of the transmit signal with the signal received by the antenna and thus creates a mixed product whose frequency corresponds to the difference between the frequency of the current transmit signal and the frequency of the received signal.
When a radar echo is received from a located object, the frequency of the mixed product is a function of the signal runtime and thus of the distance from the object, but, due to the Doppler effect, it is also a function of the relative speed of the reflecting object. Every located object is thus distinguished in the spectrum in which the mixed product is formed on each modulation ramp as a peak situated at the frequency that is a function of the distance and the relative speed. By comparing the frequency positions of peaks originating from the same object on modulation ramps having differing slope, the distance and relative speed of the object can then be determined.
Raindrops or spray are also, in this sense, “objects” that cause a weak but still detectable peak in the spectrum when they are not too far from the radar sensor, for example at distances up to approximately 10-50 m. When there is greater precipitation, these peaks are added together in the frequency range that corresponds to the above-named distance range (the relative speed of the raindrops in the radio direction at elevation 0° can mostly be disregarded) to form a background signal, the so-called rain clutter. The power of this rain clutter is thus a measure of the presence and strength of precipitation.
If, however, “real” objects, such as vehicles driving in front, are also located in the location range of the radar sensor, causing a significantly more pronounced peak in the spectrum, the power in this peak must be left out of account in the evaluation of the power of the rain clutter.
If, however, a large number of real objects are located at different distances, as can happen for example when traveling in alleys or tunnels, or when traveling directly alongside a truck, the corresponding object peaks are distributed so tightly in the spectrum that the detection threshold is raised, with the consequence that it is no longer possible to reliably distinguish between the objects and the rain clutter. In addition, the effectiveness of this method is limited by disturbing factors such as installation tolerances, temperature, and aging.